In EP 1 625 964 A2, EP 1 284 212 A1, DE 100 51 847 A1 and EP 1 586 478 A1 inlet fittings with protective additions are described, which have means to prevent a fuel tank of a Diesel motor from being improperly filled with gasoline. Known inlet fittings of the cited patents encompass a profiled, shape-fit connector (hereinafter, designated as “connector”) and a therewith movement-coupled blocking agent. The inlet fittings move between an idle position, during which elements of the fittings extend into the filling pipe, and a released, or free-flow, position during which the penetration of a pistol type feed nozzle therein is permitted. A connector is inserted at a lesser distance within the filling pipe than is the blocking agent, however, the connector invades to such an extent, that the Diesel nozzle can move the inlet fittings radially outward into a free-flow setting whereby the filling pipe then receives intake therefrom, but conversely, the entrance of a nozzle designed for gasoline is rejected, due to the fact that the gasoline nozzle would be of a smaller diameter.